1. Field of the Art
This disclosure is generally related to instrument calibration systems for gyroscopes, and more specifically to correction of gyroscopic drift for gimbal-stabilized platforms mounted on military-grade and other vehicles.
2. Background
Modern warfare has evolved to value precision weapons, which typically limit collatoral damage while reducing ordinance mass required to be delivered to the front lines. Precision weapons include smart bombs, guided missiles and artillery shells, sniper guns, and lasers, among other kinetic and nonkinetic arms. For optimal use, precision weapons require precise targeting so that they can hit their intended targets.
Electro-optical sensors have filled a vast niche required by militaries for precision targeting, as well as reconnaissance, threat warning, and positive target identification. They can be purely passive, relying on sunlight, starlight, or thermal emissions to image a target. Passive image systems do not require illumination from the sensor. Thus, passive image systems can remain hidden from the target as well as others nearby. Light and compact enough to fit on vehicles, they can be taken to a battlefront and employed in combat situations. However, small disturbances to electro-optical sensors, such as vibrations or unsteady hands, are magnified when looking across long distances.
Gimbal stabilization helps electro-optical systems stay pointed at a particular target in the distance, compensating for vibrations as well as movement of a vehicle. Such gimbal-stabilized electro-optical systems have become prevalent on modern military land vehicles and aircraft. A gimbal stabilized platform or other section supports the electro-optical system (or weapon system) and automatically keeps the system pointed at the target without an operator having to make any control inputs.
Gimbal stabilization is often performed using gyroscopes. Gyroscopes, whether mechanical, ring laser, or fiber optic, typically have some gyroscopic drift. That is, they tend to sense a slight amount of rotation even when they are still, owing to imperfections in bearings, laser mirrors, etc. Gyroscopic drift is often in one direction for awhile with a constant rate over time; however, the direction and rate are notoriously unpredictable and can change depending on external factors, such as temperature.
A gyroscope can be mounted on a movable, pointable section of a gimbal, such as an elevation axis of a pan-tilt gimbal. When the gimbal is moved in azimuth, elevation, etc., the gyroscope senses rotation and outputs the angle directions to which the movable section points. The gyroscope can be part of a closed loop system that stabilizes the gimbaled platform.
Common gimbal systems for heavy equipment include pan-tilt mechanisms. A pan-tilt mechanism includes a fixed base, a pan assembly, and a tilt assembly. The pan assembly is attached to the fixed base and rotates around a vertical axis. The tilt assembly is mounted to the pan assembly and rotates around the vertical axis with the pan assembly. The tilt assembly also rotates around a horizontal axis to point up or down. Thus, the tilt assembly can be panned (i.e., turned in azimuth) and tilted (i.e., raised in elevation).
A “half-yoke” assembly includes a gimbal with a pan assembly that cantilevers a tilt assembly on one side. A “full-yoke” assembly includes a gimbal with a pan assembly that supports a tilt assembly between two ears.
Smaller gimbal systems can include ball head gimbals. In such a design, a ball head (i.e., a sphere) is held captive by supports. The supports can be fingered, opposite each other like a vise, or formed as a monolithic socket. A platform supported by the sphere can be rotated up and down or side to side as desired.
In any gimbal design, a direction to which the gimbal points can be determined by two angles: an azimuth angle and an elevation angle. These angles can be measured with resolvers or encoders in motors that drive the azimuth and elevation angles or are otherwise embedded in the gimbal assembly.
An inertial navigation unit (INU) can be used to determine the geographic location of a vehicle (e.g., latitude, longitude, height above mean sea level). With the geographic location vehicle and a position of the target with respect to the vehicle, a geographic location of the target can be easily determined. The geographic coordinate can be used for precision weapon delivery from another vehicle to the target.
Even with all of this advanced equipment, there is a constant desire for perfection. There is a need in the art for better, more precise, more accurate gimbal assemblies and stabilization algorithms.